Overdraft limits restrict how much into the negative your bank account can go. Credit ratings are designed to help paint a story about risk, for lenders to consider when deciding how much money you can be loaned, and at what interest rate.

Both of those things existed in the early 2000s, but if the risk of a loan can (appear to) be shifted onto someone else, banks can and will issue bigger and riskier loans to people, and will reward the individual people selling the loans personally.